Profile milling is a process of progressively removing a predetermined amount of stock material from a workpiece in order to contour a machined part or to create holes designed in the part. A vertical-spindle machine has the tool mounted in a permanent vertical position. The tool is mounted horizontally on a horizontal-spindle machine. In two and one-half (21/2) axis milling the axis of the cutting tool is maintained in a fixed direction. The workpiece, which is firmly fixed on the machine table, moves freely in both the X and Y directions as the X and Y saddles slide along their respective way. Curved cuts are accomplished as both saddles slide along the two ways simultaneously.
For 21/2 axis milling, no rotation about any axis is allowed. Thus free motion in the X and Y directions provides two milling axes, while the additional 1/2 axis of motion is provided by allowing the tool to move in successive steps toward and/or away from the table, i.e. in the Z direction. Once a Z-level for milling is established, all machine motion, following prescribed tool paths, is planar, meaning that Z is fixed. After removing prescribed stock material on the fixed Z-level, the tool is moved (usually) more deeply into the workpiece (stock material), and a new set of tool paths is initiated. In general, tool paths on each Z-level differ from those on previous levels. Appropriate incrementations in Z-levels for milling depend on the geometry of the part and surface quality required. Equations exist to guide the incrementation. During 21/2 axis profile milling, material is removed along the shoulder of the tool as well as at the bottom. The implications are twofold: the tool paths must remain at least the distance of the tool radius from the part or gouging will result, and secondly that the geometry of the part must be such that no under cutting is required. Many parts satisfy this geometric requirement and 21/2 axis profile milling is the most common type of part milling performed.
A tool path is the path or geometry that a tool must follow to mill or contour a designed part. General tool paths can be fit with as many linear and/or circular segments as required to meet required tolerance of accuracy to be made suitable for machines with limited modes of motion.
Computer generated control of the milling machine activity is officially called Numerical Control (NC) or Computer Numerical Control (CNC). NC replaces manual tool pathing and process control most effectively when the sequence of operations is complex and must be performed rapidly. With reliable NC, less dependence is placed on the operator's knowledge and experience with idiosyncrasies of the machine tool or of the part being milled, and the result is higher machine utilization and better parts. Automation also results in greater operator safety as all decisions about tool pathing, and tool retraction and control are under Numerical Control. Automated and optimized tool pathing allows designers to exercise more freedom in design of parts. Production of prototype parts becomes easier and faster, allowing rapid design review and design changes.
Parts being profiled or contour machined today are more complex, varied and intricate than those previously attempted. Today's parts have significant amounts of material to be removed, may have long, thin ribs or webs, and often have holes. Accurate, efficient tool paths are essential for success. What is needed is a system that, given geometric models of the workpiece, the part, avoidance and containment regions, acceptable scallop height, and desired tool radius, the system will automatically produce and optimize tool paths on suitably chosen successive Z-levels, generate machine code, and produce a tape or file containing all milling instructions required for milling the part.